Protein synthesis is a fundamental regulatory step in gene expression. Most translational control occurs at the level of initiation of polypeptide synthesis. Stress, such as growth factor depletion, heat shock, and virus infection, rapidly inhibits protein synthesis through phosphorylation of the alpha subunit of the eukaryotic translation initiation factor 2 (eIF-2alpha). The most ubiquitous eIF-2alpha kinase known in mammalian cells is the double-stranded RNA-dependent serine/threonine kinase (PKR). As part of the interferon response, PKR expression is induced, and upon activation by double-stranded RNA, it mediates its well-established anti-viral activities. However, the applicant's preliminary data show that PKR-deficient mouse models display protection from poliovirus-induced neuropathy. In addition, disruption of the PKR signaling pathway can interfere with apoptotic responses in the absence of a viral infection. Thus, it is important to consider other functions of PKR. The preliminary data suggest that PKR signaling for the apoptotic response requires phosphorylation of eIF-2alpha, the best characterized PKR substrate. It is the applicant's hypothesis that inhibition of protein synthesis by PKR phosphorylation of eIF-2alpha inhibits cell growth and induces apoptosis. The aim of this proposal is to elucidate the mechanism of PKR-induced apoptosis. In order to test this hypothesis, the applicant will study the cellular responses in PKR knock-out mice, in mice that express a trans-dominant negative mutant of PKR, and in mice that express a non-phosphorylatable eIF-2alpha. To test the hypothesis, the applicant proposes two specific aims: 1) to identify the relationship between PKR and gene products that control apoptosis, and 2) to elucidate the role of eIF-2alpha phosphorylation in apoptosis using a mouse model harboring a S51A mutation in the eIF-2alpha gene.